The present invention relates to a device for protecting input signals of an electrical circuit against interference pulses of a higher frequency.
Such a device is also known from the DE-PS No. 28 32 766. DE-OS No. 2832766 discloses a protection device comprising a capacitor which substantially determines the time constant of a low-pass filter which acts against higher-frequency interference signals or interference pulses. The low-pass filter substantially comprises a transistor which is switched via the capacitor into its conductive state when an interference pulse occurs and accordingly loads the source of interference at a low impedance. Low-frequency signals do not lead to an actuating of the transistor, so the overall arrangement acts like a low-pass filter.
A substantial disadvantage of this known protection device consists in that a very large capacitor is required, which does not readily permit a monolithic integration of the protection device, because the capacitors which can be realized in integrated circuits at a reasonable expense are in the order of magnitude of 100 pF. Moreover, currents can only be realized reasonably down to the order of magnitude of 0.5 uA. For even smaller currents, the limit is set by blocking currents, which sharply increase at high temperatures in particular. In practice, therefore a limit is set for the realization of integrated low-pass filters in monolithic integration technology. After this limit, one must work with a large capacitor which is to be connected from the outside to the remaining integrated portion of the protection device for reasons relating to costs and layout.